1. Field of the Invention
The invention relates to a printing unit of a web-fed rotary press having a plurality of press units and at least one impression cylinder, each press unit having a printing cylinder and a transfer cylinder for printing single color images on a web.
2. Description of the Related Art
Printing units of web-fed rotary presses, in particular of newspaper presses, have a plurality of press units, each press unit comprising a transfer cylinder, a forme cylinder and an inking unit, and optionally a damping unit. The forme cylinders are also called plate cylinders and the transfer cylinders are also called blanket cylinders. Furthermore printing units of this type can have impression cylinders, it being possible for one impression cylinder to interact with one or more transfer cylinders of different press units. The transfer cylinders are also called satellite cylinders. In addition to printing units which have impression cylinders of this type, printing units are also known which do not have impression cylinders, the transfer cylinders of two press units rolling on one another in printing units of this type without impression cylinders. Accordingly, a web-fed rotary printing unit having a plurality of press units comprises a plurality of forme cylinders and a plurality of transfer cylinders and optionally one or more impression cylinders. The present invention relates to printing units which have a plurality of forme cylinders, a plurality of transfer cylinders and at least one impression cylinder. Printing units of this type are also called satellite printing units. Satellite printing units of web-fed rotary presses usually have four press units and accordingly four forme cylinders and four transfer cylinders and one or two impression cylinders. Satellite printing units of this type having four press units and one impression cylinder are called nine-cylinder printing units; printing units having four press units and two impression cylinders are called ten-cylinder printing units, in contrast.
In a satellite printing unit of this type, single-color printed images for preferably a plurality of printed pages are applied in the region of each press unit in one printing color onto the printing material which is moved through between the transfer cylinders and the or each impression cylinder. For this purpose, in what is known as a thrown-on position of the printing unit, a pressure has to be applied firstly between the forme cylinders and the transfer cylinders of the press units and secondly between the transfer cylinders and the respective impression cylinders, this pressure being provided by pivoting in or throwing on the transfer cylinders to the respective forme cylinder and to the respective impression cylinder. For this purpose, the transfer cylinders are usually mounted eccentrically and positioned relative to the respective forme cylinder and impression cylinder in such a way that a center point of the transfer cylinders does not lie on a connecting line between center points of the corresponding forme cylinders and impression cylinders. Rather, a connecting line of a center point of a transfer cylinder with the center point of the corresponding impression cylinder encloses an angle between 5° and 40° with the connecting line of the center points of the corresponding forme cylinders and impression cylinders.
The transfer cylinders, the forme cylinders and the or each impression cylinder of printing units of this type are subjected to deflection on account of their inherent weight and on account of the pressure or contact forces. This deflection depends, in particular, on the design of the cylinders, relatively thin cylinders being subjected to a relatively pronounced deflection. As a result of the deflection of the transfer cylinders and forme cylinders and impression cylinders, a considerable deviation in the circumferential register can be produced between the single-color printed images which are to be printed in the different press units of the printing unit, which deviation impairs the print quality. As has already been mentioned, these deviations in the circumferential register are particularly pronounced when relatively thin cylinders are present.
In relatively thin cylinders, the ratio of printed pages or printed images which are positioned next to one another axially to printed pages or printed images which are positioned behind one another in the circumferential direction is great, as, for example, in 6/2 cylinders in which six printed pages or printed images are positioned next to one another in the axial direction and in each case two printed pages or printed images are positioned behind one another in the circumferential direction. The same is true for 3/1 cylinders, 4/1 cylinders or else 6/1 cylinders.